Build with ASan and other memory checkers

[davidben]

C++ tooling is much much better now than it used to be. There have been a lot of problems with Conscrypt that would have been immediately caught by tools like ASan.

[davidben]

I played with this a little yesterday. Progress so far:

• If you build and link with -fsanitize=address, the resulting .so file has a dependency on the ASan runtime. (See #286)

• New enough clang (I use the one from Chromium) support a -shared-libasan flag which makes the .so file include a dependency on the runtime, but you need to set LD_LIBRARY_PATH to the relevant directory for it to pick it up.

• If you do that, it complains the ASan runtime isn't first in the link order. That can be suppressed with ASAN_OPTIONS=verify_asan_link_order=0, but then it complains that the address space ASan would be using is already taken, presumably by Java.

• I found some references which suggests one should LD_PRELOAD the runtime instead, which should avoid the need for -shared-libasan I think? But if I do that on gradlew, java crashes in free. If I could instead pass that only to the test incantation, there's a chance that might work better? But I haven't yet gotten a full manual command out of Gradle to toy with. Someone who understands that buildsystem better may be better-suited here.